The present invention generally relates to the field of laminate coatings and more particularly, is directed to a method and apparatus for bonding or cladding a layer of material onto a substrate with minimal bulk heating of the substrate.
There are a variety of techniques known in the prior art for bonding or cladding a layer of dissimilar material onto a substrate. Known techniques which produce enhanced bonding between the coating and the substrate include:
1. Static processes; PA1 2. Thermal spray processes; PA1 3. Conventional and explosive welding; and PA1 4. Laser, microwave and electron beam joining.
Static process techniques include such conventional approaches as adhesive joining, cladding and brazing. Simple adhesives are, however, impractical in many cases while conventional cladding usually involves the simultaneous application of intense heat and pressure. This is a slow process which results in considerable bulk heating of the substrate. In brazing, a heat-absorbing brazing material is placed between the coating and the substrate. Conventional brazing again causes substantial heating of the substrate and often produces a relatively weak bond.
Thermal spray processes are widely used to apply coatings, particularly in cases where the coating does not bond well to the substrate. The typical thickness of such coatings is 100 microns. The process typically involves heating the coating material in powder form to molten temperatures and blasting it onto the substrate at speeds of 500 m/s. Thus, both heat and pressure (in the form of the droplet kinetic energy) is involved in the bonding process. Although this technology has been available commercially for decades, and the coating quality can be excellent, the capital cost of devices such as plasma sprayers and detonation guns is substantial. The processing cost of such coatings commonly exceeds a dollar per square inch. Also, the process is not appropriate if the applied layer is already in the form of a plate or film (i.e., for cladding) . The terms "coating" and "cladding" although sometimes used interchangeably, more properly refer to different application processes. Cladding refers to bonding a plate or film to a workpiece, while coating refers to a bonding process that also changes the form of the material being bonded (e.g., applying liquids or powdered solids to a workpiece). Preferably, an improved bonding process will work for both coating and cladding applications.
If the coating and substrate are metallurgically compatible or "weldable", then conventional welding techniques may be used. This process is sometimes referred to as fusion hardfacing. Fusion hardfacing cannot be used in cases where the substrate cannot be heated or the materials are not weldable. Difficult to weld materials such as titanium can often be bonded to other materials by an explosive welding process. A small amount of high explosives is used to accelerate a flyer plate which drives the materials together. However, explosive welding is of ten not suitable to high volume processing, and there is little control over the process.
Many bonding processes involving advanced heat sources such as lasers, microwave, and electron beams have been proposed, but few are used commercially. Both lasers and continuous (DC) electron beams have been used for fusion hardfacing. Lasers are relatively inefficient and must rely on thermal conduction to heat the coating-substrate interface unless the coating is transparent to the laser. Microwaves are similar to electron beams in that they can produce rapid heating into the depth of the coating layer, provided that the coating is not highly conducting. Although these processes produce controlled, localized heating, they do not in general produce the additional pressure which is often required for a strong bond between dissimilar materials. In addition, most are not suitable for high volume industrial applications.